In observing an image on a CRT under a bright ambient light, the ratio of the intensity of light emitted from a fluorescent substance to the intensity of light reflected from the face of the fluorescent substance as the face plate decreases so that the contrast of image reduces with increasing the brightness of an ambient light. Various methods have been proposed to overcome this problem and some methods have been commercially practiced.
One of these practical methods uses a colored glass wherein the intensity of reflected light of an ambient light is reduced using a neutral density glass as the face plate or the front face filter on the plate. The lower the light transmittance of the neutral density glass, the higher the contrast of the image, but at the same time, the brightness (luminance) of the image is decreased. Thus, the neutral density glass currently used has a light transmittance of 50% or more.
Attempts have been made to minimize the decrease of the image brightness by bonding the neutral density filter or another filter, but the bonding portion tends to break by heat or mechanical shocks and the bonding interface causes undesired reflection or scattering of light.
It is known that the Nd.sup.3+ ion in a glass has a sharp and intense light absorption at wavelengths of 530 nm and 585 nm. U.S. Pat. No. 4,288,250 discloses a contrast enhancement filter for CRT display devices composed of phosphor P-43 comprising a glass containing a relatively large amount of Nd.sub.2 O.sub.3 and incorporated therein coloring components such as Cr.sub.2 O.sub.3 and CuO based on the finding that the glass containing a relatively large amount of Nd.sub.2 O.sub.3 has a transmission peak with a very small half-width at a wavelength near 550 nm and this peak is very close to the luminous peaks of P-43.